1. Field of the Invention
The present invention relates to a rotation prevention mechanism for a fluid displacement apparatus.
2. Description of the Prior Art
Scroll type fluid displacement apparatuses are known in the art. For example, U.S. Pat. No. 5,102,315, which are incorporated herein by reference, describes a typical apparatus.
Referring to FIG. 1, a fluid displacement apparatus in accordance with the prior art is shown in the form of a scroll type refrigerant compressor unit 100. Compressor unit includes a compressor housing 10 having a front end plate 11 and a cup-shaped casing 12 attached to an end surface of front end plate 11.
An opening 111 is formed in the center of the front end plate 11 to permit passage of a drive shaft 13. An annular projection 112 is formed in a rear end surface of front end plate 11, which faces cup-shaped casing 12. Annular projection 112 is concentric with opening 111. An outer peripheral surface of annular projection 112 extends into an inner wall of the opening of cup-shaped casing 12. Cup-shaped casing 12 is fixed on the rear end surface of front end plate 11 by a fastening device, for example, bolts and nuts, so that the opening of cup-shaped casing 12 is covered by front end plate 11. An O-ring 14 is placed between the outer peripheral surface of annular projection 112 and the inner wall of the opening of cup-shaped casing 12 to seal the mating surface of front end plate 11 and cup-shaped casing 12. Front end plate 11 has an annular sleeve 15 integrally projecting from the front end surface thereof which surrounds drive shaft 13 and defines a shaft seal cavity.
Drive shaft 13 is rotatably supported by sleeve 15 through a bearing 17 located near the front end of sleeve 15. Drive shaft 13 has a disk 18 at its inner end which is rotatably supported by front end plate 11 through a bearing 19 located within opening 111 of front end plate 11. A shaft seal assembly 20 is coupled to drive shaft within the shaft seal cavity of sleeve 15.
A magnetic clutch includes a pulley 21, an electromagnetic coil 23, and an armature plate 25. The pulley 21 is rotatably supported by a bearing 22 which is located on an outer surface of sleeve 15. The electromagnetic coil 23, which surrounds sleeve 15, is supported by a support plate 24 in an annular cavity of pulley 21. The armature plate 25 is elastically supported on the outer end of drive shaft 13 which extends from sleeve 15. In operation, drive shaft 13 is driven by an external drive power source, for example, a vehicle engine, through a rotation force transmitting device, such as the magnetic clutch described above.
A fixed scroll 26, an orbiting scroll 27, a driving mechanism for orbiting scroll 27, and a rotation reventing and thrust bearing device for orbiting scroll 27 are located within an inner chamber of cup-shaped casing 12. The inner chamber is formed between the inner wall of cup-shaped casing 12 and front end plate 11.
Fixed scroll 26 includes circular end plate 261, a wrap or spiral element 262 affixed to or extending from an end surface of circular end plate 261, and a plurality of internally threaded bosses 263 axially projecting from the other end surface of circular plate 261. An axial end surface of each boss 263 is seated on the inner surface of an end plate 121 of cup-shaped casing 12 and fixed by bolts 28. Thus, fixed scroll 26 is fixed within the cup-shaped casing 12. Circular plate 261 of fixed scroll 26 divides the inner chamber of cup-shaped casing 12 into a discharge chamber 30 and suction chamber 29. A seal ring 132 is located between the outer peripheral surface of circular plate 261 and the inner wall of cup-shaped casing 12. A hole or discharge port 264 is formed through circular plate 261 at a position near the center of spiral element 262. Discharge port 264 is connected between the central fluid pockets of the spiral element 262 and discharge chamber 30.
Orbiting scroll 27 also includes a circular end plate 271 and a wrap or spiral element 272 affixed to or extending from one end surface of circular end plate 271. Spiral element 272 of orbiting scroll 27 and spiral element 262 of fixed scroll 26 interfit at an angular offset of 180 degrees and a predetermined radial offset. At least one air of fluid pockets are thereby defined between spiral elements 262 and 272. Orbiting scroll 27, which is connected to drive mechanism and to the rotation preventing and thrust bearing device, is driven in an orbital motion at a circular radius R.sub.o by drive shaft 13 to compress fluid passing through compressor unit 100. Generally, radius R.sub.o of orbital motion is given by the following formula: EQU R.sub.o =(pitch of spiral element)-2.times.(wall thickness of spiral element)!/2
The spiral element 272 is radially offset from spiral element 262 of fixed scroll member 26 by distance R.sub.o. Thus, orbiting scroll 27 undergoes orbital motion of a radius R.sub.o upon rotation of drive shaft 13.
Drive shaft 13, which is rotatably supported by sleeve 15 through bearing 17, is connected to disk 18. Disk 18 is rotatably supported by front end plate 11 through bearing 19 disposed within opening 111 of front end plate 11. A crank or drive in 33 axially projects from an axial end surface of disk 18 at a position which is radially offset from the center of drive shaft 13. Circular plate 271 of orbiting scroll 27 has a tubular boss 273 axially rejecting from the end surface opposite the surface from which spiral element 272 extends. A discoid or short axial bushing 34 fits into boss 273 and is rotatably supported therein by a bearing, such as a needle bearing 35. Bushing 34 has a balance weight 341 which has the shape of a semi-disk or ring radially connected to bushing 34 along a front surface thereof. An eccentric hole 342 is formed in bushing 34 at a position radially offset from the center of bushing 34. Drive in 33 fits into eccentric hole 342. Bushing 34, which is driven by the revolution of drive in 33, rotates within bearing 35.
The rotation of orbiting scroll 27 is prevented by a rotation preventing and thrust bearing device positioned between the inner wall of the housing 10 and circular plate 271 of orbiting scroll 27 and around boss 273 of orbiting scroll 27. As a result, orbiting scroll 27 orbits while maintaining its angular orientation relative to fixed scroll 26.
Referring to FIGS. 2, 3, 4 and 5, rotation preventing and thrust bearing device is provided with an annular fixed race 130, an annular orbital race 131, and bearings, such as a plurality of balls 137. Annular fixed race 130 is secured to axial end surface 113 of front end plate 11 by a plurality of fixed pins 138. Orbital race 131 is secured to end surface 271a of circular plate 271 of orbiting scroll 27 by a plurality of fixed pins 139. Annular fixed race 130 and annular orbiting race 131 each have a plurality of pockets 130a and 131a, respectively, in an axial direction preferably formed by a press working process. The number of pockets in each race 130 and 131 is equal. Annular fixed race 130 and annular orbiting race 131 face each other at a predetermined axial clearance. The radius of each pocket 130a of annular fixed race 130 is about the same as that of each pocket 131a of orbital race 131. Pockets 130a correspond generally in location to pockets 131a, i.e., each pair of pockets facing each other have the same pitch, and the radial distance of each set of pockets from the centers of their respective races is about equal.
Further, annular fixed race 130 includes a plurality of openings 130b formed on a circumference thereof at an angular interval. Front end plate 11 includes a pair of holes 114 formed thereon at the angular interval corresponding to the angular interval of opening 130bof annular fixed race 130. Annular fixed race 130 is secured to axial end surface 113 of front end plate 11 by fixed pins 138, such that fixed in 138 inserts into hole 114 of front end plate 11 through opening 130b. Furthermore, annular fixed race 130 may be secured to front end plate 11, such that radial inner end of axial end surface overlies radial edge of fixed race 130 by use of caulking.
Annular orbiting race 131 includes a plurality of openings 131b formed on a circumference thereof at an angular interval. Circular end plate 271 includes a pair of holes 275 formed thereon at the angular interval corresponding to the angular interval of opening 131b of annular orbital race 131. Annular orbital race 131 is secured to circular end plate 271 of orbiting scroll 27 by fixed pins 139, such that fixed in 139 inserts into hole 275 of orbiting scroll 27 through opening 131b of orbital race 131. Further, pockets 130a and 131a of annular fixed and orbital races 130 and 131, respectively, includes bottom lane portions axially offset from one end surface of annular fixed and orbital races 130 and 131, respectively. Centers of pockets 130a and 131a are formed on the circle of radius R about radial centers O.sub.1 and O.sub.2, respectively. A diameter of bottom portion of pockets 130a and 131a is designed to be substantially equal to radius R.sub.0 which is the orbital radius of orbiting scroll 27. Center O.sub.1 of fixed race 130 and center O.sub.2 of orbital race 131 are designed to be coincident with center O.sub.a of front end plate 11 and center O.sub.b of orbiting scroll 27, respectively.
The operation of the compressor is described below. As the orbiting scroll 27 orbits, a plurality of line contacts between spiral elements 262 and 272 moves toward the center of the spiral elements along the surface of the spiral elements. The fluid pockets, which are defined by spiral elements 262 and 272, also move toward the center with a consequent reduction in volume and compression of the fluid in the fluid pockets. The fluid or refrigerant gas, which is introduced into suction chamber 29 from an external fluid circuit through inlet port 31 (not shown), is drawn into the fluid pockets formed between spiral elements 262 and 272 from the outer end of the spiral elements. As orbiting scroll 27 orbits, fluid in the fluid pockets is compressed, and the compressed fluid is discharged into discharge chamber 30 from the central fluid pocket of the spiral elements through discharge port 264. The fluid then is discharged to the external fluid circuit through an outlet port (not shown).
When orbiting scroll 27 is driven by rotation of drive shaft 13, the center O.sub.2 of orbital race 131 orbits about a circle of radius R.sub.o. However, a rotation force, i.e., moment, which is created by the offset of the acting point of the reaction force of compression and the acting point of the drive force, acts on orbiting scroll 27. This reaction force tends to rotate the orbiting scroll 27 about the center O.sub.2 of orbiting race 131. Thus, the locus of the contact points of each ball 137 on each pair of pockets 130a and 131a generally outlines a circle having radius R.sub.o, i.e., the traveling radius of each of ball 137 with respect to the axial end surface of fixed race 130 and orbital race 131 is defined by R.sub.o. The rotation of orbiting scroll 27 is prevented by balls 137, each of which makes contact with walls of pockets 130a and 131a during operation while the angular relationship between fixed scroll 26 and orbiting scroll 27 is maintained. Moreover, the axial load from orbiting scroll 27, which is caused by the reaction force of the compressed gas, is carried by fixed race 130, orbital race 131, and balls 137.
In general, it is desired that a sealing force at the line contacts between spiral elements 262 and 272 be sufficiently maintained in a scroll type compressor, because the fluid pockets are defined by the line contacts between the two spiral elements which are intermitted together, and the line contacts shift along the surface of the spiral elements toward the center of spiral elements by the orbital motion of scroll member, to thereby move the fluid pockets to the center of the spiral elements with consequent reduction of volume, and compression of the fluid in the pockets. If contact force between the spiral element becomes too large in maintaining the sealing line contacts, wear of spiral elements increases. In view of this, the contact force of both spiral elements must be suitably maintained.
The operation of the rotation preventing/thrust bearing device is illustrated, in art, in FIG. 6. The center O.sub.2 of orbital race 131 is shown at the right side of the center O.sub.1 of fixed race 130, and the rotation direction of drive shaft 13 is clockwise as indicated by arrow "A." When orbiting scroll 27 is driven by the rotation of drive shaft 13, center O.sub.2 of orbital race 131 orbits about a circle of radius "R.sub.o " (together with orbiting scroll 27). However, an offset of the acting point of drive force, acts on orbiting scroll 27. This reaction force tends to rotate orbiting scroll 27 in a clockwise direction about center of orbital race 13 1. But, as shown in FIG. 6, balls 137 are laced between the corresponding pockets 130a and 131a of fixed and orbital races 130 and 131, respectively. In the position shown in FIG. 6, the interaction between the nine balls at the to of the rotation preventing/thrust bearing device and the edges of the pockets 130a and 131a prevents the rotation of orbiting scroll 27.
In the assembling of fixed race 130 and orbital race 131 to front end plate 11 and orbiting scroll 27, respectively, fixed race 130 and orbital race 131 may be eccentrically placed with respect to center O.sub.a of front end plate 11 and center O.sub.b of orbiting scroll 27, respectively. In other words, when fixed race 130 is re-assembled to front end plate 11, center O.sub.1 of fixed race 130 may not be coincident with center O.sub.a of front end plate 11, and when orbiting scroll 27 is pre-assembled to orbital race 131, center O.sub.2 of orbital race 131 may not be coincident with center O.sup.b of orbiting scroll 27.
As a result, when the orbiting orbital race 131 and orbiting scroll 27 are assembled to fixed race 130 and front end plate 11 by inserting boss 273 of orbiting scroll 27 into bushing 34 through bearing 35, center O.sub.2 of orbital race 131 may not lie on a circle of radius R.sub.o formed about center O.sub.1 of fixed race 130 because of the eccentricities between fixed race 130 and front end plate 11 and between orbiting race 13 1 and orbiting scroll 27. The offset is caused, in art, by dimensional errors in the manufacturing and assembling of fixed and orbital races 130 and 131.
The eccentricities described above reduce the ability of the compressor to maintain suitable contact between both spiral elements and cause balls 137 to run on edges of pockets 130a or 131a of races 130 or 131, respectively. As a result, the eccentricities reduce compression efficiency of the compressor and increase abrasion between fixed race 130 and orbital races 131.
An assembler may inspect for eccentricities by measuring the distortion of the orbiting locus of orbiting scroll 27, or after assembly, a sample of the compressors may be overhauled to observe abrasion vestiges between spiral elements 262 and 272 of fixed scroll 26 and orbiting scroll 27, respectively. Such production inspections, however, are complex to perform, consume much time, and do not provide precise measurements of the eccentricities.